Multicellular organisms replace worn-out cells through cell division. However, in some animals, cell division eventually halts, and the cell is then referred to as senescent. Senescent cells deteriorate and die, causing the body to age. Cells stop dividing because the telomeres, protective bits of DNA on the end of a chromosome, become shorter with each division and eventually can no longer protect the chromosome. Cancer cells, on the other hand, are "immortal." An enzyme called telomerase allows them to continue dividing indefinitely.